Porous MgO-stabilized CaO-based powders/pellets via a citric acid-based carbon template for thermochemical energy storage in concentrated solar power plants
Introduction
The impact of rising anthropogenic CO2 emissions, mainly derived from fossil fuel combustion, which are causing climate change, is undeniable. At the 21st Conference of Parties (COP 21), 195 participating countries committed to restrict global-mean temperature rise to well below 2 °C by 2100 [1]. To achieve such a challenging target, renewable energy technologies [2], particularly solar energy [3], must be installed on a massive scale to steadily replace fossil fuels. Due to its intrinsic intermittency, directly incorporating solar energy-derived power into an electricity grid makes the existing energy network unstable and insecure [4]. Concentrated Solar Power (CSP) [5] offers a feasible integration opportunity for large-scale thermal energy storage (TES) to produce electricity on demand, thus eliminating the intermittency problem of solar energy [6], [7]. However, current commercial CSP-TES systems based on molten salts suffer from limited thermal stability, high freezing temperature, corrosion issues, and high cost when compared with conventional fossil fuel power plants [8], [9].
A potential alternative to TES involves developing thermochemical energy storage (TCES) which operates at high temperatures, where sensible heat from solar irradiation can directly drive a reversible endothermic chemical reaction [10]. The resulting products from this reaction are stored separately to produce power when required [11]. TCES has an inherently larger energy density, higher working temperature, and the possibility of storing energy indefinitely with negligible thermal losses compared to TES [12]. A diverse set of materials, such as metal oxides [11], sulfates [13], [14], hydroxides [15], [16], [17], and carbonates [18], [19], have been proposed for TCES. Among them, one of the most promising candidates is calcium carbonate based on the calcium looping (CaL) process [20], because of its advantages including a high working temperature (~850 °C), high energy density (~3.2 GJ/m3), and high thermal conductivity [21]. This allows the employment of highly efficient power cycles to obtain electric efficiencies of 40–50%, plus the use of a well-documented process that is already applied in the cement and lime industry. An added, important advantage is that cheap non-toxic natural Ca-based minerals (limestone) (~$10/ton), are widely available [21]. This process relies on the cyclic carbonation/calcination reaction of CaCO3 [22] as follows:
The schematic diagram of CSP-CaL process recently proposed in Fig. 1. It consists of a solar calciner, a carbonation reactor, a CO2 compression-storage system, two reservoirs for CaO and CaCO3 storage, two heat exchangers, and a power unit. After calcination of CaCO3 in the calciner using concentrated solar energy, the sensible heat of CaO and CO2 is recovered through two heat exchangers and these products are stored independently. On demand, CaO and CO2 are circulated into the carbonator wherein heat is released by the exothermic carbonation reaction. This heat is transported by the CO2, in excess, through different powder cycles to a gas turbine where electricity is generated while the effluent CO2 is sent to storage. Significantly different from CaL as a CO2 capture process (carbonation at ~650 °C under low CO2 partial pressure of ~15 vol% and calcination at high temperature of ~950 °C under high CO2 concentration of ~85 vol%), the CaL conditions in the CaL-CSP process involve carbonation at a high temperature (~850 °C) under pure CO2 concentration to maximize the thermoelectric efficiency while calcination at 700–750 °C occurs under low CO2 partial pressures to allow the use of conventional volumetric receivers composed of low cost alloys [20].
However, progressive decay in CaO conversion occurs with sequential carbonation-calcination cycling, approaching low residual values (typically X ~ 0.07) for limestone-derived CaO after 20 cycles [23]. Such CaO deactivation is a significant disadvantage for a potential CaL system integrated in a CSP plant (CSP-CaL). According to Ortiz et al. [20], large-scale equipment is, thus, needed due to the reduced CaO conversion because of the presence of high amounts of effectively inert solids in the system. Moreover, these non-reacting solids must be conveyed, preheated and cooled through the CSP plant, leading to greatly increased loss of efficiency. It was reported that the energy density of the system would be decreased from 0.89 to 0.26 GJ/m3 as CaO residual conversion takes values from X = 0.07 to X = 0.05 [24]. Accompanying this effect, the overall thermal-to-electric efficiency would be lowered by 10% [25]. Thus, enhancing the CaO multicycle conversion is highly desirable and remains the main challenge in a CSP-CaL process.
It is well known that CaO deactivation is caused by significant sintering-induced loss of meso-porosity or pore plugging [26], [27]. This arises because the low Tammann temperature (TT) of CaCO3 (~529 °C) is below the typical operating temperature (~850 °C) [28]. Pore plugging is related to the rapid formation of a ~100 nm product layer of CaCO3 on the surface of the CaO particles that limits further CO2 diffusion into the unreacted core of the particles [29]. To address this issue, one effective strategy is to design porous CaO particles with a stable pore structure (high-TT stabilizer) [30], [31]. In particular, porous MgO-stabilized CaO is promising, since MgO does not consume active CaO, unlike materials such as SiO2 [32], Al2O3 [33], [34] and ZrO2 [35], which unfortunately produce CaxSixOz, CaxAlyOz and CaZrO3, respectively, thus reducing the available CaO. Furthermore, MgO is a cheap and environmentally benign material (e.g., in the form of dolomite). Apart from dolomite, calcium-based industrial wastes (e.g., steel slag) [36] can be another cheap and abundant source. It should be noted that preparation MgO-stabilized CaOs using organic acids have been reported widely in CaL as a CO2 capture approach [37]. However, MgO-stabilized CaOs applied in a CSP-CaL process have been rarely examined. Recently, Sánchez Jiménez et al. [38] prepared porous CaCO3-MgO composites using acetic acid-treated limestone/dolomite mixtures for high-temperature TCES. The optimized sorbent achieved a high and stable conversion (0.7 mol/mol) after 20 cycles. Unfortunately, due to its weakly acidic character, a large amount of acetic acid (8.6 mol per CaO) must be used to ensure the production of calcium/magnesium acetate precursors, making the procedure rather expensive. Moreover, these relatively large-particle-sized powders should be further examined through mechanical strength tests to verify their suitable utilization in fluidized bed reactors.
Here, we used citric acid as a new sacrificial template to prepare porous MgO-stabilized CaO-based sorbents for high-temperature TCES. Citric acid is produced abundantly from the beverage and food industries, and thus has a price comparable to acetic acid. However, unlike acetic acid treatment, a smaller amount of citric acid (0.78 mol per CaO) is needed to produce soluble calcium/magnesium precursors due to its stronger acidity, which can greatly reduce the cost of synthesis. Supposing that all the carbon components in the two types of organic acids is transformed into CO2, the amount of CO2 emissions for acetic and citric acid would be 17.2 and 4.7 mol per mol of CaO, respectively. Furthermore, a carbon template can be formed through wet and/or dry mixing of citric acid with calcium/magnesium precursors followed by pyrolysis under nitrogen. Our previous studies proved that the presence of such a carbon template not only suppressed grain growth but also moderated the segregation of Ca and Mg during the high-temperature procedure [39]. This favorable morphology leads to an improved CaO conversion in TCES. More importantly, the citric acid-based carbon template was also extended to synthesize porous MgO-stabilized CaO pellets via an extrusion–spheronization route, which avoids the further addition of a carbon source during common synthesis. This route for synthesizing pellets is dry and need only employ natural limestone-dolomite mixtures, making it easily scalable. These pellets were also examined for their mechanical strength to demonstrate their suitability for use in fluidized bed reactors. To further understand the performance of such materials, a detailed structural analysis and examination of their cyclic sorption characteristics were made to explore the underlying deactivation mechanism.
Section snippets
Sorbents
Analytical grade Ca(NO3)2·4H2O and Mg(NO3)2·6H2O were used as the calcium/magnesium-based precursors to prepare MgO-stabilized CaO as fine powders. Then citric acid monohydrate was dissolved in distilled water. Calculated amounts of calcium/magnesium nitrates with various molar ratios of Ca2+ to Mg2+ were mixed with the appropriate quantities of the acid solution to obtain 100 mL total volume. The resulting solutions were then vigorously stirred into a slurry at 80 °C, and then oven-dried at
Pure CaO powders
The cyclic heat release characteristics of two pure CaO samples using one-step and two-step calcination are compared in Fig. 2a. The initial heat release capacity of Ca9CA7-a was low, ~1.75 kJ/g sorbent, and it subsequently declined to ~1.0 kJ/g after 20 cycles. Upon two-step calcination, Ca9CA7 achieved a higher initial release capacity of Qr (2.78 kJ/g), corresponding to CaO conversion of 88% and had a relatively slower deactivation rate when compared with Ca9CA7-a. The evolution of heat
Conclusions
A simple cost-effective and scalable synthesis technique to yield highly efficient MgO-stabilized CaO sorbents was developed. The presence of citric acid served as a carbon template to suppress grain growth, moderate the segregation of Ca and Mg and improve porous structure, in turn mitigating pore-plugging and sintering. Thus, a high capacity and stable cyclic performance for MgO-stabilized CaO powders was obtained. Moreover, for dry-mixing of citric acid with limestone-dolomite mixtures, this
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Financial support for this work from the Fundamental Research Funds for the Central Universities (2018XKQYMS13) is gratefully acknowledged.
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Ke Wang is currently a visiting scholar at Cranfield University.